Touch-sensitive membrane switches have been incorporated into many electronic devices to enable operators to provide instructions to the device by selecting a corresponding horizontal and vertical coordinate location on the membrane switch. For example, membrane switches are often installed over the viewing screen of a cathode ray tube. The user of a device including such a "touchscreen" is able to operate the device by pointing to and depressing a particular location on the screen corresponding to a desired menu selection. The touchscreen then generates a voltage signal corresponding to the horizontal ("x") and vertical ("y") coordinates of that location. For such an application, the layers used to fabricate the membrane switch are transparent.
Other conventional applications for membrane switches are numeric and function keypads on diverse electronic items, such as microwaves, television sets, calculators, medical instrumentation, and various other devices. Membrane switches may be designed for manual finger or stylus depression for operation. The range of applications for membrane switches is ever increasing, as is the need for producing low-cost membrane switches.
One type of conventional membrane switch, often used for touch-sensitive screens, is the analog membrane switch. The membrane switch comprises a sandwich of top and bottom membranes with at least the top membrane being made from a flexible material. More typically, both membranes are made from flexible dielectric sheets. One surface of each membrane is coated with a semiconductive resistive layer, such as indium tin oxide ("ITO"), or a conductive layer such as gold.
Construction and operation of conventional membrane switches is well known in the art. One method of constructing an analog membrane switch is described in U.S. Pat. No. 5,228,562 to Burk. The membrane switch includes a flexible top membrane having a lower conductive surface; a lower membrane having an upper conductive surface; and an intermediate spacer disposed therebetween. The intermediate spacer includes a central rectangular aperture, and has upper and lower opposite surfaces. One or more first (y-axis) electrodes are formed on the upper surface of the intermediate spacer along a set of parallel edges, and one or more second (x-axis) electrodes are applied on the lower surface of the intermediate spacer along another set of parallel edges. The electrodes are applied, typically, by silk-screening with a conductive ink.
A random or fixed array of small raised dielectric projections is deposited on the upper surface of the lower membrane. Next, conductive adhesive is applied between the intermediate spacer and the top and bottom membranes to secure the intermediate spacer in place with the x- and y-axis electrodes in electrical contact with the top and bottom membranes, respectively. At this point, the raised dielectric projections are positioned within the rectangular aperture of the intermediate spacer, normally maintaining the lower conductive surface of the top membrane separated from the upper conductive surface of the lower membrane.
However, when the top membrane is depressed through the central aperture of the intermediate spacer, it contacts the bottom membrane between the projections. The x and y coordinate locations of this point of depression can be obtained by monitoring voltage drops across the electrodes. Typically, a uniform potential, such as 5 volts, is first applied across a first set of electrodes formed on the upper surface of the intermediate spacer while the voltage drop across the second set of electrodes on the lower surface of the intermediate spacer is monitored. This voltage corresponds to the horizontal, or "x" coordinate of the depression pointer. This arrangement is then switched, with a potential applied across the second set of electrodes and the voltage drop across the first set of electrodes being monitored to determine the vertical, or "y" coordinate. Monitoring of first and second sets of electrodes oscillates in this manner so that both the x and y coordinates of a depression point can be rapidly measured when such a depression occurs. Other voltage monitoring methods may be used to obtain similar results.
The method of producing a membrane switch as described above is advantageous in that many of the processing steps, such as application of electrodes, are performed on the intermediate spacer, reducing the opportunities for scratching or marring the fragile conductive coating formed on the top and lower membranes. However, the method is rather cumbersome because it requires application of electrodes on both upper and lower surfaces of the intermediate spacer. To this end, the intermediate spacer needs to be flipped over after one of its surfaces is applied with a set of electrodes, and the intermediate spacer's position needs to be carefully adjusted for precise alignment and secured before its other surface is applied with another set of electrodes. With such careful adjustment, however, application of electrodes on both sides of an intermediate spacer and possible misalignment of the electrodes often result in a final membrane switch product that is not reliable.